This invention relates generally to improvements in a thermostatic expansion valve for refrigeration systems and more particularly to a balanced valve assembly including in combination a piston sensing an inlet chamber pressure, one or more pusher rods communicating a capillary line pressure to a valve member and a biasing member biasing the valve member to a closed position so as to provide balanced refrigerant flow through a valve port in the expansion valve in response to changes in refrigeration system operating conditions, including refrigeration load and refrigerant pressure drop across the valve port.
Balanced thermostatic expansion valves have been used in refrigeration systems for many years to automatically control the flow of refrigerant from the high pressure side of a refrigeration system, that is, the discharge from the condenser unit. Most balanced thermostatic expansion valves presently in use incorporate a valve motor or diaphragm and operate in response to a differential pressure across the diaphragm. The differential pressure is partially a function of a superheat of a vaporized refrigerant as it leaves the evaporator or as it is commonly referred to, evaporator superheat.
Conventional refrigeration valves, such as may be found in U.S. Pat. No. 3,742,722 for a Thermostatic Expansion Valve for Refrigeration Systems, U.S. Pat. No. 3,738,573 for an Expansion Valve, and U.S. Pat. No. 2,786,336 for a Refrigerant Expansion Valve Mechanism, employ a stem associated with a valve motor or a diaphragm wherein the valve motor or diaphragm responds to a capillary pressure. The capillary pressure is generally a function of the evaporator superheat.
During operation of the refrigeration system the evaporator superheat and pressure of the refrigerant as it leaves an evaporator outlet will vary as a result of changing load conditions and a change in the refrigerant pressure drop across the valve port. A change in the discharge pressure of the refrigerant leaving the condenser, or head pressure as it is also referred to, due to the difference between summer and winter operation of the refrigeration system, as a result of a change in the ambient temperature of the condenser, may vary as much as from fifty (50) to three hundred (300) pounds per square inch.
The result of such a variation is a change in evaporator superheat and a change in a refrigerant pressure difference across the expansion valve. The change in the refrigerant pressure difference across the expansion valve changes the amount of refrigerant flow through the expansion valve and changes evaporator superheat.
An increase in condenser discharge pressure or head pressure at or above a predetermined condenser pressure limit may affect the expansion valve in a number of ways. It may result, for example, in an increased refrigerant pressure drop across the valve port and require a reduced valve opening to maintain refrigerant flow and a constant superheat at the evaporator outlet.
In most expansion valves the refrigerant flows in the same direction that the valve port opens. When the differential pressure of the refrigerant across the valve port varies there is a variation in the forces which operate the expansion valve. These forces, due to an unbalanced pressure condition within the expansion valve, add to the force corresponding to caplillary pressure on the diaphragm and are opposed by the counterforce of the biasing member, such as a valve spring, tending to close the valve port and reduce refrigerant flow. A balance between the forces acting on the diaphragm plus unbalanced pressure across the valve port and the biasing member determines the amount of refrigerant flow through the expansion valve and the evaporator superheat.
The thermostatic expansion valve of the present invention solves these and other problems in a manner not disclosed in the known prior art.